Method for stabilizing titanium alloys against hydrogen pickup and stabilized titanium alloy produced thereby

ABSTRACT

1. A TITANIUM ALLOY CAPABLE OF BEING CHEMICALLY PROCESSED IN A SOLUTION CONTAINING HYDROGEN IONS WITHOUT PICKING UP AN EXCESSIVE QUANTITY OF HYDROGEN IONS WITHOUT PICKSISTING ESSENTIALLY OF TITANIUM, ALUMINUM, VANADIUM AND TIN AND BEING KNOWN AS TI-6A1-6V-2SN, SAID ALLOY BEING IN THE BETA OR ALPHA-BETA- PHASES, SAID ALLOY HAVING RELATIVELY LOW STRENGTH AND HIGH DUCTILITY AS A RESULT OF HAVING BEEN ANNEALED BY HEATING TO AT LEAST ABOUT 1300*F. BUT LESS THAN THE SOLUTION TREATING TEMPERATURE AND BY THEREAFTER COOLING RELATIVELY SLOWLY, SAID ALLOY THEREAFTER HAVING BEEN MAINTAINED BETWEEN ABOUT 800*F. AND ABOUT 1000*F. FOR AT LEAST APPROXIMATELY ONE HOUR TO STABILIZE IT AGAINST HYDROGEN PICKUP.

United States Patent 3,846,188 METHOD FOR STABILIZING TITANIUM ALLOYS AGAINST HYDROGEN PICKUP AND STABI- LIZED TITANIUM ALLOY PRODUCED THEREBY Robert G. Werkema, St. Louis County, and Robert E. Newcomer, St. Louis, Mo., assignors to McDonnell Douglas Corporation, St. Louis, M0. N0 Drawing. Filed June 9, 1972, Ser. No. 261,504

Int. Cl. CZZE 1/18; (3231? 1/04 US. Cl. 148-133 4 Claims ABSTRACT OF THE DISCLOSURE To prevent titanium and its alloys from picking up excessive hydrogen during chemical processing, such as during etching or pickling, or in the presence of corrosive substances, the titanium metal is heated to between 800 F. and about 1000 F. for 1 to 4 hours prior to being subjected to the chemical processing or the corrosive substances. The heat treatment does not significantly affect the mechanical properties of the titanium metal, for the temperature at which it takes place is too low, but it does prevent the chemical phenomenon known as hydrogen embrittlement which would otherwise occur upon subject ing the titanium to the chemical processing or the corro sive substances.

BACKGROUND OF THE INVENTION This invention relates to titanium, and more particularly to a method for stabilizing titanium and its alloys against hydrogen pickup and a stabilized titanium alloy produced thereby.

Titanium and its alloys are more difiicult to machine than other metals and titanium fabricators often resort to so-called chemical milling procedures to effect removal of the metal. Chemical milling is a controlled etching process and the most common etchants contain hydrofluoric acid. Hydrofiuoric acid, by itself, is not suitable for etching titanium alloys which contain significant quantities of the beta phase, since these alloys tend to pick up hydrogen from the acid during dissolution and become extremely brittle. The phenomenon is known as hydrogen embrittlement. It has been determined that for most aircraft applications the maximum amount of hydrogen a titanium alloy may contain is about 180 p.p.m. Since titanium alloys consisting of both alpha and beta phases possess physical characteristics far superior to alloys in the alpha phase, the former alloys are much more prevalent than the latter and indeed are used for practically all aerospace fabrications.

Other chemical processing operations to which titanium alloys are subjected also produce hydrogen embrittlement. Pickling is an example of such a process. Indeed, merely subjecting the alloys to a corrosive environment in which hydrogen ions exist usually will cause the alloys to pick up hydrogen and become brittle.

The practicality of current chemical processing procedures resides in the fact that the chemical processing solutions contain ingredients which retard the tendency of the alloys to pick up hydrogen. In the case of some titanium alloys the formulation of the chemical etching solutions are extremely effective in retarding hydrogen pickup, and the responsive alloys pick up little or no hydrogen. The titanium alloy known as Ti-6Al-4V (consisting essentially of about 6% aluminum, 4% vanadium, and the remainder titanium) is an example of such an alloy. On the other hand, other titanium alloys tend to pick up relatively large amounts of hydrogen, notwithstanding the presence of the additional ingredients in the chemical processing solutions for the purpose of retarding hydrogen pickup.

3,846,188 Patented Nov. 5, 1974 The titanium alloy known as Ti-6Al-6V-2Sn (consisting essentially of about 6% aluminum, 6% vanadium, 2% tin, and the remainder titanium) represents an example of a titanium alloy which does not respond to solution formulations designed to retard hydrogen pickup.

SUMMARY OF THE INVENTION One of the principal objects of the present invention is to provide a process for treating titanium and its alloys such that the metal will not pick up excessive quantities of hydrogen during subsequent chemical processing operations, such as chemical milling and pickling, or in the presence of corrosive substances. Another object is to provide a process of the type stated which does not have deleterious effects on the titanium metal. A further object is to provide a process which is simple and does not require specialized equipment. An additional object is to provide a titanium alloy which does not pick up excessive quantities of hydrogen when it is chemically milled or subjected to corrosive substances. These and other objects and advantages will become apparent hereinafter.

The present invention resides in a process whereby titanium and its alloys are stabilized against hydrogen pickup in the presence of chemical treatment solutions or corrosive substances by heating the titanium metals to between about 800 F. and about 1000 F. prior to subjecting them to the chemical treatment solutions or to the corrosive substances. The invention also resides in the stabilized alloy produced by the process.

DETAILED DESCRIPTION Broadly speaking, the process of the present invention involves heating a titanium metal, which is usually a titanium alloy, to between 800 F. and about 1000 F. and maintaining the alloy at the elevated temperature for between one and four hours. Thereafter, the metal is cooled. Usually, the titanium metal is in the alpha-beta phase, for it is these metals which are the most useful to the aerospace industry. After reaching room temperature, the metal may be chemically processed without picking up excessive quantities of hydrogen from the chemical processing solution. The heat treatment stabilizes the titanium metal aaginst hydrogen pickup, and although the metal will probably pick up some hydrogen during subsequent chemical processing, the amount of hydrogen existing therein will be below the generally accepted maximum of 180 p.p.m. Clearly, the stabilized metal picks up substantially less hydrogen than a titanium metal which is not subjected to the heat stabilization process of the present invention.

Preferably, the titanium metal is heated to about 900 F. and maintained at that temperature for two hours. Times shorter than one hour may result in insufiicient stabilization; times longer than 4 hours tend to provide negligible further improvement and are undesirable from a manufacturing standpoint.

The manner in which the metal is cooled from the elevated temperature of between 800 F. and about 1000 F. is immaterial. It may be air cooled, furnace cooled, or quenched.

The temperature to which the titanium metal is heated, that is 800 F. to about 1000 F., is somewhat less than the temperature at which mechanical properties of titanium alloys are adjusted, and hence the stabilization process differs from conventional heat treatment. Indeed, the process is useful with annealed alloys for the mechanical properties of these alloys do not change significantly at temperatures as low as 800 F. to 1000 F. For example, the alpha-beta titanium alloy Ti-6Al-6V-2Sn is annealed by heating it to between 1300" F. and 1500 R, which is below the solution treating temperature, and then air cooling it. In general, to alter the mechanical properties of the alloy, it must be heated to at least 1100 F. Thus, it is apparent that the stabilization process controls an aspect of secondary material behavior, that is, hydrogen pickup during chemical processing, and is not related to conventional heat treatment procedures to control mechanical properties.

Commercial titanium alloys normally contain both the alpha and beta phase, for alpha-beta alloys have the best physical properties. While these alloys contain some hydrogen, the hydrogen content is far below the acceptable limit of 180 p.p.m. Typically, it ranges between 40 p.p.m. and 70 p.p.m. Hydrogen is practically insoluble in alpha titanium, but is much more soluble in beta titanium. Hence alpha-beta titanium containing a significant amount of the beta phase tends to pick up the hydrogen.

Fabrication processes applied to titanium alloys involve forming the simple shapes provided by producing mills into the complex configurations required in detail parts. Simple contours and generous radii bends may be accomplished at room temperature; providing compound curvatures and small bend radii requires heating the material to elevated temperatures in the range of 1200 F. to 1450" F. These elevated temperatures, and the cooling rate from these temperatures, may leave the alloy with undesirable properties. To alleviate undesirable mechanical properties, material is usually heated to temperatures in excess of 1000 F. and then air cooled. Typically it is heated to 1100 F., held at that temperature for about one hour, and then air cooled.

After the heat treatment to adjust the mechanical properties, the titanium alloy is subjected to still another heat treatment if it is to undergo chemical processing, and that heat treatment is for the purpose of stabilizing the alloy against hydrogen pickup during chemical processing. In particular, the alloy is heated to between 800 F. and about 1000 F. and held at the elevated temperature for between one and four hours. Once the alloy has cooled, it may be chemically processed and will not pick up an excessive amount of hydrogen from the solution and experience hydrogen embrittlement. Since the heat treatment for hydrogen stabilization does not take place at temperatures high enough to alter the mechanical properties of the alloy, the mechanical properties remain substantially unchanged. In other words, the heat treatment for stabilization takes place at a temperature below the heat treatment for adjusting mechanical properties.

Where heat treatments for hot working the alloy and for adjusting its mechanical properties are involved, they should precede the heat treatment to stabilize against hydrogen pickup. Stated differently, the heat treatment to stabilize against hydrogen pickup should be the last heat treatment prior to chemical processing. Where the alloy is heated to adjust its mechanical properties; it may be cooled directly from the temperature at which that heat treatment is effected into the lower temperature range for stabilization, or it may be cooled to room temperature and then reheated to the temperature range for stabilization. In either case, the alloy must remain in the stabilization range for at least one hour.

EXAMPLE I A titanium alloy known as Ti-6Al-6V-2Sn and produced by Titanium Metals Corporation of America (Timet) initially contained 77 p.p.m. hydrogen, possessed a yield strength of 148,000 p.s.i., had an ultimate strength of 160,000 p.s.i., and its elongation was 12.5%. The alloy was heated to 1350 F., held at that temperature for minutes, and while at that elevated temperature the alloy was shaped in a press. During the shaping operation the alloy lost much of its heat to the process and its temperature decreased significantly. Thereafter, the alloy was reheated to 1100 F., held at that temperature for 65 minutes, and air cooled, for the purpose of adjusting its mechanical properties. Finally, a portion of the surface area on the alloy was masked, and the alloy was immersed in chemical milling solution consisting essentially of 4 v/o hydrofluoric acid concentration), 55 v/o nitric acid (42 B. concentration), and 30 g./l. urea. The alloy remained in the solution for 30 minutes, and by the end of that time the solution had etched the unmasked area to 0.030 inches. After the chemical milling operation, the hydrogen content of the alloy was 203 p.p.m., its yield strength was 143,000 p.s.i., its ultimate strength was 155,- 000 p.s.i., and its elongation was 10%.

The same initial alloy was subjected to the foregoing heat treatments at 1350" F. and 1100" F. Subsequent to the air cooling forming part of the latter heat treatment, the alloy was heated to 900 F., maintained at that temperature for 65 minutes, and then air cooled, for the purpose of stabilizing the alloy against hydrogen pickup. The stabilized alloy was masked and etched to 0.030 inches in the same chemical processing solution for the same length of time. The etched alloy contained 136 p.p.m. hydrogen, possessed a yield strength of 148,000 p.s.i., an ultimate strength of 160,000 p.s.i., and elongation of 12%.

Thus, the additional heat treatment at 900 F. stabilized the titanium alloy against hydrogen pickup and reduced the hydrogen content of the chemically milled product from 203 p.p.m. to 136 p.p.m. The latter quantity is well within prescribed standards.

EXAMPLE II The titanium alloy Ti-6Al-6V-2Sn produced by Reactive Metals, Inc., possessed a hydrogen content of 40 p.p.m. as supplied by its producer. This alloy was chemically milled to 0.030 inches in the chemical milling solution of Example I. The alloy remained in the solution approximately 30 minutes. After the etching, the alloy possessed a hydrogen content of 230 p.p.m.

The same initial alloy, that is the alloy containing 40 p.p.m. hydrogen, was heated to 1350 F. for two hours, cooled, and then reheated to 1100 F. for another two hours to adjust its mechanical properties. Thereafter, the alloy was chemically milled to 0.030 inches for 50 minutes in the same chemical processing solution for substantially the same length of time. After the etching, the alloy contained 260 p.p.m. hydrogen.

The same initial alloy, that is the one having a 40 p.p.m. hydrogen, was heated to 1350 F. for two hours, was then reheated to 1100 F. for two hours to adjust its mechanical properties, and finally was reheated to 900 F. for two hours to stabilize it against hydrogen pickup. The alloy was thereafter chemically milled to "0.030 inches in the same chemical processing solution for substantially the same length of time. The etched alloy contained p.p.m. hydrogen, well below the acceptable limit.

EXAMPLE III The titanium alloy Ti-6Al-2Sn-4Zr-6Mo produced by Titanium Metals Corporation of America (Timet) has also been found to respond to hydrogen pickup control through thermal stabilization. As produced by the material supplier, 0.027-inch material possessed a hydrogen content of 124 p.p.m. After etching to 0.008 inches the material possessed a hydrogen content of 240 p.p.m. After stabilization at 1050 F. for two hours, etching to 0.008 inches provided material which possessed hydrogen content of only 177 p.p.m.

This invention is intended to cover all changes and modifications of the example of the invention herein chosen for purposes of the disclosure which do not constitute departures from the spirit and scope of the invention.

What is claimed is:

1. A titanium alloy capable of being chemically processed in a solution containing hydrogen ions without picking up an excessive quantity of hydrogen, said alloy consisting essentially of titanium, aluminum, vanadium and tin and being known as Ti-6Al-6V-2Sn, said alloy being in the beta or alpha-beta phases, said alloy having relatively low strength and high ductility as a result of having been annealed by heating to at least about 1300" F. but less than the solution treating temperature and by thereafter cooling relatively slowly, said alloy thereafter having been maintained between about 800 F. and about 1000 F. for at least approximately one hour to stabilize it against hydrogen pickup.

2. A method of stabilizing an annealed alpha-beta titanium alloy known as Ti-6Al-6V-2Sn so that it will not pick up excessive quantities of hydrogen in the presence of substances containing hydrogen ions, the alloy having been previously annealed at between about 1300 F. and about 1500 B, said method of stabilizing comprising: heating the annealed alloy to maintain it at a temperature between approximately 800 F. and approximately 1000 F. for at least approximately one hour.

3. A method of stabilizing an annealed alpha-beta titanium alloy known as Ti-6Al-6V-2Sn so that it will not pick up excessive quantities of hydrogen in the presence of substances containing hydrogen ions, the alloy having been previously annealed at between about 1300 F. and about 1500 F.; said method of stabilizing comprising: treating the previously annealed alloy in the temperature range of approximately 800 F. to approximately 1000 F. for an amount of time sufficient to stabilize the same against picking up excessive quantities of hydrogen and at a temperature below the annealing temperature and below the temperature at which substantial changes in the mechanical properties of the annealed alloy occur, said temperature being maintained sufficiently high to stabilize the previously annealed alloy against picking up excessive quantities of hydrogen when the stabilized alloy is thereafter subjected to a corrosive substance containing hydrogen ions.

4. A method of etching the alpha-beta titanium alloy known as Ti-6Al-6V-2Sn, the alloy having relatively low strength and high ductility as a result of having been annealed by heating to above about 1300 F. and to below the solution treating temperature and cooling relatively slowly therefrom; said method comprising: maintaining the alloy at between approximately 800 F. and approximately 1000 F. for at least about one hour to stabilize the alloy against hydrogen pickup; and thereafter placing the alloy in an etchant containing hydrogen ions.

References Cited UNITED STATES PATENTS 3,069,259 12/1962 Margolin et al. l75.5

OTHER REFERENCES Metallurgical Transactions, vol. 1, June 1970, pp. 17754777.

Alloy Digest, Filing Code Ti-52, August 1967.

CHARLES N. LOVELL, Primary Examiner US. Cl. X.R. 

1. A TITANIUM ALLOY CAPABLE OF BEING CHEMICALLY PROCESSED IN A SOLUTION CONTAINING HYDROGEN IONS WITHOUT PICKING UP AN EXCESSIVE QUANTITY OF HYDROGEN IONS WITHOUT PICKSISTING ESSENTIALLY OF TITANIUM, ALUMINUM, VANADIUM AND TIN AND BEING KNOWN AS TI-6A1-6V-2SN, SAID ALLOY BEING IN THE BETA OR ALPHA-BETA- PHASES, SAID ALLOY HAVING RELATIVELY LOW STRENGTH AND HIGH DUCTILITY AS A RESULT OF HAVING BEEN ANNEALED BY HEATING TO AT LEAST ABOUT 1300*F. BUT LESS THAN THE SOLUTION TREATING TEMPERATURE AND BY THEREAFTER COOLING RELATIVELY SLOWLY, SAID ALLOY THEREAFTER HAVING BEEN MAINTAINED BETWEEN ABOUT 800*F. AND ABOUT 1000*F. FOR AT LEAST APPROXIMATELY ONE HOUR TO STABILIZE IT AGAINST HYDROGEN PICKUP. 